1. Field of the Invention
This invention relates to a color television camera, and more particularly to a color television camera which may prevent occurrence of color shading.
2. Description of the Prior Art
Generally in a color television camera, a color separating optical system is disposed in the light path of an objective lens so that the light beam from an object may be separated into beams of red, green and blue wavelength ranges by the color separating optical system and these three beams may impinge on image pick-up devices for respective color channels so as to form red, green and blue images of the object on the respective light receiving surfaces which are photosensitive. The color separating optical system usually employed has comprised a prism or a glass plate having evaporated thereon a so-called dichroic layer which is capable of reflecting almost all of the light of a particular wavelength range and transmitting almost all of the light of the other wavelength ranges. In order that the light beam from the object may be separated into light beams of three colors as described above, it is only required, for example, that a dichroic layer capable of reflecting the blue light and transmitting the lights of the other wavelength ranges and a dichroic layer capable of reflecting the red light and the lights of the other wavelength ranges be successively disposed in an inclined relationship with the optical axis of the objective lens.
The wavelength selecting characteristic of the dichroic layer depends on the angle of incidence of light. More specifically, as the angle of incidence of light is varied, the spectral distribution of the reflected light, hence that of the transmitted light is varied. Therefore, unless the camera is specially designed, the phenomenon known as color shading will occur on the image display surface of the television receiver. For convenience of description, this will now be discussed with reference to the conventional color television camera as shown in FIG. 1 of the accompanying drawings. In FIG. 1, an objective lens 1 is disposed in opposed relationship with an object O which is to be displayed as an image for viewing on the image display surface of a television receiver. Designated by 2 is an exit pupil of the objective lens. First second dichroic mirrors 3, 4 have color separating surfaces 3' and 4' (hereinafter referred to as the first and second color separating surfaces) are coated respectively with a dichroic layer capable of reflecting light of the blue wavelength range and transmitting lights of the other wavelength ranges, and a dichroic layer capable of reflecting light of the red wavelength range and transmitting lights of the other wavelength ranges. As shown, the first and second dichroic mirrors 3 and 4 are disposed in the back-focus space of the objective lens 1 in an inclined relationship with respect to the optical axis A of the objective lens in such a manner that the reflecting axes RA.sub.3 and RA.sub.4 of the two dichroic mirrors lie in a common plane (plane of the drawing sheet) and at the opposite sides with respect to the optical axis A of the objective lens. Designated by 5.sub.B is a blue channel image pick-up tube having a light receiving surface 5.sub.B ' disposed at such a position as to receive the blue light from the object reflected by the first dichroic mirror 3 via a mirror 7 for reducing the size of the camera by bending the light path, to form thereon the image of the object passed through the objective lens 1. Designated by 5.sub.G and 5.sub.R are green and red channel image pick-up tubes, respectively. The light receiving surface 5.sub.G ' of the red channel image pick-up tube is disposed at such a position as to receive the green light beam passed from the object through the second dichroic mirror 4 and to form thereon the image of the object passed through the objective lens 1. The light receiving surface 5.sub.R ' of the red channel image pick-up tube is disposed at such a position as to receive the red light beam from the object reflected by the second dichroic mirror 4 and to form thereon the image of the object passed through the objective lens 1. Thus, the blue, green and red images 6.sub.B, 6.sub.G and 6.sub.R of the object are formed on the light receiving surfaces 5.sub.B ' , 5.sub.G ' and 5.sub.R ' . The image pick-up tubes 5.sub.B, 5.sub.G and 5.sub.R respectively scan these images 6.sub.B, 6.sub. G and 6.sub.R to generate electrical signals corresponding to the respective images, namely, the distribution of light quantity on the light receiving surfaces. In the light path between the first dichroic mirror 3 and the light receiving surface 5.sub.B ' , and in the light path between the second dichroic mirror 4 and the light receiving surfaces 5.sub.G ' , 5.sub.R ' , there are disposed trimming filters 8.sub.B, 8.sub.G and 8.sub.R having the characteristics to be described later, so as to regularize the spectral distribution on the respective light receiving surfaces.
Now, as shown, B.sub.c, G.sub.c and R.sub.c define the center points in the light receiving surfaces 5.sub.B ' , 5.sub.G ' and 5.sub.R ' of the image pick-up tubes, namely, the points, at which the center point O.sub.c of the object O is focused; B.sub.l, G.sub.l and R.sub.l at the points, at which the points O.sub.l in the object O (in FIG. 1, the lower point of the object on the plane of the drawing sheet) is focused; and, B.sub.u, G.sub.u and R.sub.u define the points whereat the point O.sub.u in the object (in FIG. 1, the upper point of the object on the plane of the drawing sheet) is focused. In FIG. 1, the principal ray in the light beam travelling from the objective lens 1 toward the center point G.sub.c and the opposite extremities G.sub.l and G.sub.u of the light receiving surface 5.sub.G ' for the green channel is represented by the lines connecting the center P of the exit pupil of the objective lens 1 to the points G.sub.c, G.sub.u and G.sub.l, namely, the lines PG.sub.c, PG.sub.l and PG.sub.u. Assuming that .theta. is an angle formed between the principal ray PG.sub.c and the principal respective rays PG.sub.l and PG.sub.u, .alpha. is the angle of incidence of the principal ray PG.sub.c on the color separating surface 3' of the first dichroic mirror 3, and .beta. the angle of incidence of the principal ray PG.sub.c on the color separating surface 4' of the second dichroic mirror 4. The angles of incidence of the principal ray PG.sub.l on the first and second color separating surfaces 3' and 4' are .alpha.-.theta. and .beta.+.theta., respectively, and the angles of incidence of the principal ray PG.sub.u on the first and second color separating surfaces 3' and 4' are .alpha.+.theta. and .beta.-.theta., respectively. Apparently, these angles of incidence have the relationship .alpha.+.beta.&gt;.alpha.&gt;.alpha.-.theta. and .beta.+.theta.&gt;.beta.&gt;.beta.-.theta..
The first dichroic mirror 3 which reflects almost all of the light of the blue wavelength range depending on the magnitude of the angle of incidence and transmits almost all of the lights of the other wavelength ranges exhibits the characteristic as illustrated in FIG. 2A, and the second dichroic mirror 4 which reflects almost all of the light of the red wavelength range and transmits almost all of the lights of the other wavelength ranges exhibits the characteristic as illustrated in FIG. 2B. In FIGS. 2A and 2B, the ordinate and the abscissa, respectively, the transmission factor and the wavelength, and the broken line curve indicates the transmission factor for the ray travelling from the point P to the points B.sub.u, G.sub.u and R.sub.u, the solid line curve indicates the transmission factor for the ray travelling from the point P to the points B.sub.c, G.sub.c and R.sub.c, and the chain line curve indicates the transmission factor for the ray travelling from the point P to the points B.sub.l, G.sub.l and R.sub.l. On the other hand, the trimming filters 8.sub.B, 8.sub.G and 8.sub.R shown in FIG. 1 have the characteristics of transmitting the blue, green and red light, respectively, as illustrated in FIGS. 3A, 3B and 3C. Considering the spectral distributions on the light receiving surfaces of the blue, green and red channel image pick-up tubes, it is seen from FIGS. 2A and 2B and FIGS. 3A, 3B and 3C that the curves representing such distributions are as illustrated in FIGS. 4A, 4B and 4C.
FIGS. 4A, 4B and 4C, the ordinate represents the quantity of light which has reached the light receiving surface of each image pick-up tube, while the abscissa represents the wavelengths, and the broken line curve indicates the spectral distribution at the points B.sub.u, G.sub.u and R.sub.u, the solid line curve indicates the spectral distribution at the points B.sub.c, G.sub.c and R.sub.c, and the chain line curve indicates the spectral distribution at the points B.sub.l, G.sub.l and R.sub.l.
Considering the distribution of the relative light quantity of light on the light receiving surfaces 5.sub.B ', 5.sub.G ' and 5.sub.R ' of the respective channel image pick-up tubes, when they pick up an image emitting a uniform intensity of white light from the entire surface thereof, it is seen in FIGS. 4A, 4B and 4C that the curves representing such distribution have the tendencies as indicated by solid lines in FIGS. 5A, 5B and 5C, respectively. In each of these Figures, the ordinate represents the relative quantity of light and the abscissa represents the line passing from the point whereat the point O.sub.l on the object is focused on the light receiving surface of the pick-up tube to the point whereat the point O.sub.u on the object is focused. The term "relative quantity of light" herein used means the ratio of the quantity of light at a spatial point in the beam impinging on the light receiving surface of the pick-up tube to a referential quantity of light which is the light quality at any given spatial point in said beam when the pick-up tube picks up the image of an object emitting a uniform intensity of white light from the entire surface thereof.
It is apparent from FIGS. 5A, 5B and 5C that if the object to be photographed is a white one, when the electrical signals generated in accordance with the quantities of light from the white object reaching the center points B.sub.c, G.sub.c and R.sub.c of the light receiving surfaces 5.sub.B ', 5.sub.G ' and 5.sub.R ' are so adjusted by known pick-up tube signal processing means that an exact white image is formed at the center of the television receiver, no white image will be formed on the opposite sides of the television receiver but colored images will appear there. This will be seen by comparing intensities of the signals obtained at the points B.sub.l, G.sub.l and R.sub.l on the light receiving surfaces 5.sub.B ', 5.sub.G ' and 5.sub.R ' which correspond to the point O.sub.l on the object. As is clear from FIGS. 5A, 5B and 5C, the blue signal obtained at the point B.sub.l and the red signal obtained at the point R.sub.l are greater in intensity than the green signal obtained at the point G.sub.l, on account of which the image corresponding to the O.sub.l side of the object is tinged in light purple on the television receiver. Likewise, the blue signal obtained at the point B.sub.u and the red signal obtained at the point R.sub.u are weaker in intensity than the green signal obtained at the point G.sub.u, so that the image corresponding to the O.sub.u side of the object is tinged in light green on the television receiver. Such color irregularity on the television receiver is called the color shading. This color shading also occurs when the beam for focusing a reference pattern image on the light receiving surfaces of the respective image pick-up tubes without causing it to pass through an objective lens is color-separated to effect control of the light beam from a light source for bias illumination, the registration of the pick-up tube signal processing device, and the white balance. The color shading resulting from the bias light will hereinafter be described as an example.
As is well-known, there are pick-up tubes which, unless specially designed, cause an afterimage to be formed on the television receiver when an object is photographed under a low intensity of illumination. In order to suppress such afterimage phenomenon, it has usually been practised to illuminate the light receiving surface of the pick-up tube with a predetermined intensity of light at all times. This is called the bias illumination.
In the apparatus of FIG. 1, in order to bias-illuminate each channel image pick-up tube, a light beam for bias illumination is thrown upon the color separating surface 3' of the first dichroic mirror 3 in a direction symmetrical with the direction of incidence of the light beam from the object relative to the color separating surface 3'. Designated by 9 is the source lamp which emits white light containing lights of various wavelengths. The light emitted from the lamp 9 is diffused by a diffusing plate 10. Designated by 11 is a condenser lens disposed with the optical axis thereof intersects with the optical axis of the object lens 1. The optical axis of the condenser lens is integrated with the optical axis of the lens 1 by the first dichroic mirror 3 disposed at the intersection between the optical axes of the condenser lens 11 and the lens 1. The condenser lens 11 focuses the image of the diffusing plate 10 or of the space adjacent thereto on the light receiving surface of each pick-up tube. The light beam passed through the condenser lens 11 reaches the blue channel image pick-up tube 5.sub.B through the first dichroic mirror, and also reaches the green and red channel pick-up tubes 5.sub.G and 5.sub.R through the first and the second dichroic mirrors 3 and 4, respectively. Fromthe characteristics of the first and second dichroic mirrors 3 and 4 illustrated in FIGS. 2A and 2B, and from the characteristics of the trimming filters 8.sub.B, 8.sub.G and 8.sub.R illustrated in FIGS. 3A, 3B and 3C, the spectral distributions of the bias light on the light receiving surfaces 5.sub.B ', 5.sub.G ' and 5.sub.R ' of the blue, green and red channel image pick-up tubes are as shown in FIGS. 6A, 6B and 6C, respectively. In these Figures, the broken line curves indicate the spectral distribution at the points B.sub.u, G.sub.u and R.sub.u on the light receiving surfaces, the solid line curves indicate the spectral distribution at the points B.sub.c, G.sub.c and R.sub.c, and the chain line curves indicate the spectral distribution at the points B.sub.l, G.sub.l and R.sub.l. (It should be noted here that the first dichroic mirror 3, for example, does not transmit therethrough 100% of the light of the red and green wavelength ranges but reflects extremely small part of such lights. Thus, the light beam from the bias light source reflected by the first dichroic mirror 3 and directed to the second dichroic mirror 4 contains therein the lights of the red and green wavelength ranges, although the light of the blue range occupies the major proportion in this light beam.) It is seen from FIGS. 6A, 6B and 6C that the relative quantity of the bias light on the light receiving surfaces 5.sub.B ', 5.sub.G ' and 5.sub.R ' of the blue, green and red channel pick-up tubes exhibits the tendencies as indicated by the solid lines in FIGS. 7A, 7B and 7C. As already described in connection with FIGS. 5A, 5B and 5C, the distribution of the quantity of the bias light on the blue channel light receiving surface 5.sub.B ' differs in tendency from that on the green or the red channel light receiving surface 5.sub.G ', 5.sub.R '. Therefore, if each channel pick-up tube is bias-illuminated by the arrangement as shown in FIG. 1, there will still occur the color shading. As also described previously, in order to project and focus a reference pattern image on the light receiving surface of each image pick-up tube, the diffusing plate 10 in FIG. 1 may be used as the reference pattern and the lens 11 as the projecting lens. Again in this case, the color shading will likewise occur.
In the past, the above-described color shading was eliminated by electrically correcting the signal or signals generated by one or two of the blue, green and red channel image pick-up tubes. For example, in the apparatus of FIG. 1, the elimination of the color shading resulting from the light beam from the object is accomplished by correcting the electrical signal generated by the green channel image pick-up tube 5.sub.G so that the level of the signal obtained at the G.sub.u side of the light receiving surface 5.sub.G ' is lower than the level of the signal obtained at the G.sub.l side. By doing so, the distribution of the relative quantity of light of the light beam which is incident on the light receiving surface 5.sub.G ' is similar to that indicated by the broken line in FIG. 5B and thus, the color shading may be eliminated.
The foregoing method, in which an electrical correction is made to the signal from the pick-up tube, is disadvantageous in that it leads to a complicated construction of the signal processing circuit. Further, the signal(s) to be corrected is in advance level-corrected at a predetermined proportion corresponding to each point on the light-receiving surface and therefore, as the absolute quantity of light incident on the light receiving surface varies, there occurs a great variation in the difference between the absolute value of the gradient of the level of the corrected signal with respect to the transverse direction of the light receiving surface and the absolute value of the gradient of the level of the uncorrected signal with respect to the transverse direction of the light receiving surface. Thus, if the quantities of light incident on the image pick-up tubes are fluctuated, the colors on the display surface of the television receiver will also be varied so that it will be impossible to sufficiently eliminate the color shading merely by electrically correcting the signal(s) from the image pick-up tube(s). Further, in the image pick-up device contained within the bias illumination system or within the reference pattern projecting system as shown in FIG. 1, that the tendency of distribution of relative quantities of light on the light receiving surfaces 5.sub.B ' and 5.sub.G ' of the blue and green channel image pick-up tubes differs from the tendency of distribution in the relative light quantity with respect to the bias-illuminating light, as is apparent from FIGS. 5A and 5B and FIGS. 7A and 7B. In such a case, it is extremely difficult to eliminate the color shading by electrically correcting the signals from the image pick-up tubes.